Class D audio amplifier and method for compensation of power supply voltage influence on output audio signal in class D audio amplifier

ABSTRACT

In a class D electroacoustic amplifier ( 1 ) without feedback loop containing a supply voltage source ( 8 ), an amplifier low-pass filter ( 14 ), a power stage ( 2 ) controlled by a pulse width modulated signal, a saw-shaped voltage generator ( 4 ) and a comparator ( 3 ), to one of which inputs an audio signal is sent, while its second input is connected to the adder ( 6 ) of the compensation circuit of supply voltage influence on the output audio signal, to which a voltage from a reference voltage source is sent, a low-pass filter ( 9 ) and a high-pass filter ( 10 ) are connected to the supply voltage source ( 8 ), and the reference voltage source ( 12 ) is connected to an inverting circuit ( 11 ), whose input is connected to the low-pass filter ( 9 ) output, while the high-pass filter ( 10 ) output and the output of the inverting circuit ( 11 ) are connected to a multiplier ( 7 ), whose output is connected to the input of another multiplier ( 5 ), whose second input is connected to the saw-shaped voltage generator ( 4 ), and the multiplier ( 5 ) output is connected to one input of the adder ( 6 ) whose second input is connected to the saw-shaped voltage generator ( 4 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a National Stage Application of International Patent ApplicationNo. PCT/PL 03/00006, with an international filing date of Jan. 18, 2003,which is based on Polish Patent Application No. P-351783, filed Jan. 21,2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a class D electroacoustic amplifier,i.e. a class D audio amplifier, and a method for compensation of powersupply voltage influence on an output audio signal in the class D audioamplifier.

2. Brief Description of the Background of the Invention Including PriorArt

One of the features of class D electroacoustic amplifiers is making useof a carrier signal apart from the audio signal. One of these amplifiersis the one known from the U.S. Pat. No. 4,182,992, which consists of,among others, two transistors and a diode.

In turn, from the U.S. Pat. No. 4,178,556 there is known a class Damplifying circuit that contains a modulating circuit to modulate anaudio signal and a carrier signal.

There is also known, from the U.S. Pat. No. 6,300,825 an amplifier thatmakes use of pulse width modulation of a signal, containing a comparatorand an integrating circuit, which in order to increase the coefficientof rejection of supply voltage fluctuations, was supplemented withanother integrating circuit.

From the Polish patent description P-325207 there is known a circuit forcompensation of the constant component of the output voltage, powered bya stabilised voltage that generates a voltage compensation signal. Thissignal is added in an adder to a voltage signal of a triangular wavegenerator and in this way it changes the constant component of theoutput signal of the generator. The changed constant componentinfluences the value of the duty cycle coefficient of the rectangularimpulses generated at the output of the comparator, compensating to adegree the supply voltage fluctuations.

SUMMARY OF THE INVENTION

1. Purposes of the invention

It is an object of this invention to provide a device and a method forincreasing quality of audio signals at the output of audio amplifiers.

This and other objects and advantages of the present invention willbecome evident from the description which follows.

2. Brief Description of the Invention

According to the present invention, in a class D electroacousticamplifier, i.e. a class D audio amplifier, without feedback loop thatcontains a supply voltage source, an amplifier low-pass filter, a powerstage controlled by a pulse width modulated signal, a saw-shaped voltagegenerator and a comparator, to one of which inputs an audio signal issent, while its second input is connected to the adder of thecompensation circuit of supply voltage influence on the output audiosignal, to which a voltage from a reference voltage source is sent, alow-pass filter and a high-pass filter are connected to the supplyvoltage source, and the reference voltage source is connected to aninverting circuit, whose input is connected to the low-pass filteroutput, while the high-pass filter output and the output of theinverting circuit are connected to a multiplier, whose output isconnected to the input of another multiplier, whose second input isconnected to the saw-shaped voltage generator, and the multiplier outputis connected to one input of the adder, whose second input is connectedto the saw-shaped voltage generator.

Preferably, the output signal v₀(t) of the inverting circuit sent to themultiplier input, which is a modified constant of the supply voltage, isexpressed by a formula v₀(t)=k₁×V_(DCref)/[k₂×v_(i)(t)], where V_(DCref)is the voltage of the reference source, v_(i)(t) is a slow-changingsignal on the low-pass filter output, and the coefficient k₁ε<0.5; 2.0>and the coefficient k₂ε<0.2; 1.5>.

Preferably, the output signal of the multiplier, which is the errorsignal e(t), sent to the multiplier, is expressed by the formulae(t)=k₃×v₀(t)×v_(ii)(t), where v₀(t) is a modified supply voltageconstant, v_(ii)(t) is a fast-hanging signal on the high-pass filteroutput, and the coefficient k₃ε<0.8; 10.0>.

Preferably, the output signal V_(CM)(t) of the adder, which is thecorrected carrier wave signal, sent to one input of the comparator, isexpressed by the formula V_(CM)(t)=k₄×V_(C)(t)×[1/k₅+e(t)], whereV_(C)(t) is a high frequency carrier wave generated by the generator,e(t) is the error signal, and the coefficient k₄ε<0.2; 1.5> and thecoefficient k₅ε<0.2; 3.0>.

The object of the invention is also a method of compensation of supplyvoltage influence on the output audio signal in an electroacousticamplifier, which contains a saw-shaped signal generator and a comparatormaking use of pulse width modulation, and which is powered from a powersupply, and to whose input an audio signal is sent, and whose secondinput is connected to an adder of a compensation circuit of supplyvoltage influence on the output audio signal, to which a voltage from areference voltage source is sent, from the power supply source afast-changing signal v_(ii)(t) is extracted and a slow-changing signalv_(i)(t), which is inverted and multiplied by the a value of a referencesupply voltage V_(Dcref), which results in an output signal v₀(t), whichthen is multiplied by a fast-changing signal v_(ii)(t), which results inan error signal e(t), which then is multiplied by a saw-shaped signalV_(C)(t) from the generator, and the resulting signal is added to asaw-shaped signal V_(C)(t) and as a corrected carrier wave V_(CM)(t) issent to one of the inputs of the comparator, which makes use of pulsewidth modulation, and to its second input the audio signal is sent.

The novel features which are considered as characteristic for theinvention are set forth in the appended claims. The invention itself,however, both as to its construction and its method of operation,together with additional objects and advantages thereof, will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The object of this invention is shown in implementation examples on theenclosed drawings, where

FIG. 1 shows a block diagram of a class D electroacoustic amplifier witha system for compensation of influence of supply voltage fluctuations,

FIGS. 2 and 3 show the result of computer simulation with thecompensation system present and the supply voltage of 27 V,

FIGS. 4 and 5 show the result of computer simulation without thecompensation system present and with the supply voltage of 27 V,

FIGS. 6 and 7 show the result of computer simulation with thecompensation system present and with the supply voltage of 40 V,

FIGS. 8 and 9 show the result of computer simulation without thecompensation system present and with the supply voltage of 40 V.

DESCRIPTION OF INVENTION AND PREFERRED EMBODIMENT

The solution of the compensation system that is presented above can beapplied in any desired system that contains a discrete class D audioline, and in the description it is supplemented with an electroacousticamplifier. A class D electroacoustic amplifier 1, i.e. a class D audioamplifier, without feedback loop, shown in FIG. 1, contains a comparator3, to whose input an audio signal is sent, and a generator 4, a powerstage 2, using pulse width modulation, a supply voltage source 8, anamplifier low-pass filter 14, and a loudspeaker device being the load ofthe amplifier 1. In the shown amplifier 1, an audio input signal 21 andan output audio signal 28 are not included in the feedback loop andcoefficient of supply voltage fluctuation influence rejection is 0 dB.So to reduce the supply voltage influence on output audio signal, theelectroacoustic amplifier 1 has been supplemented with a compensationsystem containing a low-pass filter 9 and a high-pass filter 10, both ofwhich are connected to a power supply voltage source 8.

The reference voltage source 12 of the compensation system is connectedto an inverting circuit 11, whose input is connected to the low-passfilter 9 output. The high-pass filter 10 output and the invertingcircuit 11 output are connected to a multiplier 7, whose output isconnected to a multiplier 5 input, whose second input is connected to asaw-shaped or triangular-shaped voltage generator 4, while themultiplier output is connected to one of the adder 6 inputs, whosesecond input is connected to a saw-shaped or triangular-shaped voltagegenerator 4.

In the presented electroacoustic amplifier 1, with the compensationcircuit of supply voltage influence on an audio output signal, the inputsignal 21 is sent to the ‘-’ input of the comparator 3, where thecomparison of the audio signal 21 with the carrier signal 26 V_(C)(t),which is a carrier wave of high frequency in the range of 40 kHz to 1MHz, takes place.

In this system, the voltage from the power supply source 8 is sent tothe low-pass filter 9 and the high-pass filter 10. From the high-passfilter 10 output, a fast-changing signal 23 v_(ii)(t), which is theseparated variable component of the supply voltage, is sent to one ofthe multiplier 7 inputs, while the slow-changing signal v_(i)(t), whichis the separated constant component of the supply voltage, from thelow-pass filter 9 is sent to the inverting circuit 11 input. Moreover,the voltage V_(DCref) of the reference voltage source 12 is connected tothe inverting circuit 11. Then, the signal 24, given by the formulav₀(t)=k₁×V_(DCref)/[k₂×v_(i)(t)], from the inverting circuit 11 outputis sent to the multiplier 7 input, where the multiplying of this signalwith the fast-changing signal 23 from the high-pass filter 10 outputtakes place, and the received error signal, expressed by the formulae(t)=k₃×v₀(t)×v_(ii)(t), from the multiplier 7 input is sent to one ofthe multiplier 5 inputs. In the multiplier 5, the multiplying of thecarrier signal 26 V_(C)(t) from a saw-shaped or triangular-shapedvoltage generator and the modified constant component 25 e(t) frommultiplier 7, is performed. Then, the signal from the multiplier 5 isadded in the adder 6 to the carrier signal 26. The received signal 27,which is the corrected high-frequency carrier wave, expressed by theformula V_(CM)(t)=k₄×V_(C)(t)×[1/k₅+e(t)], from the adder 6 output issent to the ‘+’ input of the comparator. With the use of the multiplier5, the adder 6, the multiplier 7, the filter 9, the filter 10, theinverting circuit 11 and the reference voltage source 12, a correctedcarrier wave V_(CM)(t) is generated, by which it is possible to maintaina constant envelope of the audio output signal sent to the loudspeakerdevice 13. The coefficients in the formulas take the values from theranges k₁ε<0.5; 2.0>, k₂ε<0.2; 1.5>, k₃ε<0.8; 10.0>, k₄ε<0.2; 1.5>,k₅ε<0.2; 3.0> and k₆ε<0.2; 3.0>.

The presented system solves the problem of a low coefficient ofrejection of influence of supply voltage fluctuations, called the PowerSupply Rejection Ratio coefficient (PSRR), when the amplifier has nofeedback loop. In this system, in order to increase the Power SupplyRejection Ratio coefficient, signals distorting the source voltage 8signal are generated without the use of the audio signal. Thus, theaudio signal is not modified directly, but indirectly through amodification of a carrier signal. In the presented system, thecompensation of the constant component fluctuations takes place throughthe multiplier 5, the adder 6, the low-pass filter 9, the invertingcircuit 11, and the compensation of the variable component fluctuationstakes place through the multiplier 5, the adder 6, the high-pass filter10, the inverting circuit 11 and the reference voltage source 12.

Further figures show the results of a computer simulation of theamplifier. FIG. 2 shows a signal 31 at the output of the power stagewith the power supply influence compensation system present. The signal31 is amplitude-modulated with a variable component of a 3 V amplitudeand a frequency 33 of 1 kHz, while the frequency 32 of the audio signalis 5 kHz and the supply voltage is 27 V. FIG. 3 shows a sinusoidalsignal with a frequency 34 of 5 kHz at the output with a load of 8 Ω.

In turn, FIG. 4 shows a signal 37 at the output of the power stagewithout the power supply influence compensation system present. Thissignal is also amplitude-modulated by a variable component of a 3 Vamplitude and a frequency 36 of 1 kHz, while the frequency 35 of theaudio signal is 5 kHz. FIG. 5 shows a sinusoidal signal with a frequency38 of 5 kHz at the output with a load of 8 Ω. The envelope of the outputwave is visibly modulated and the frequency 39 of amplitude fluctuationsis 1 kHz.

FIG. 6 shows a signal 41 at the output of the power stage with the powersupply influence compensation system present and with values of voltageschanged in comparison with signal 31 in FIG. 2. The signal 41 is alsoamplitude-modulated by a variable component of a 7 V amplitude and afrequency 43 of 1 kHz, while the frequency 42 of the audio signal is 5kHz and the power supply voltage is 40 V. FIG. 7 shows a sinusoidalsignal with a frequency 44 of 5 kHz at the output with a load of 8 Ω, aswell.

FIG. 8 shows a signal 45 at the output of the power stage without thepower supply influence compensation system present. The signal,similarly to the one shown above (FIG. 5), is amplitude-modulated by avariable component of a 7 V amplitude and a frequency 46 of 1 kHz, whilethe frequency 45 of the audio signal is 5 kHz. FIG. 9 shows a sinusoidalsignal with a frequency 48 of 5 kHz at the output with a load of 8 Ω.The envelope of the output wave is visibly modulated and the frequency49 of amplitude fluctuations is 1 kHz.

The compensation system of power supply influence on output audio signalfor the class D electroacoustic amplifier, presented above, solves to asignificant degree problems of stability of the amplifier, includingproblems of setting a phase margin.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofamplifiers differing from the types described above.

While the invention has been illustrated and described as embodied inthe context of a class D audio amplifier, it is not intended to belimited to the details shown, since various modifications and structuralchanges may be made without departing in any way from the spirit of thepresent invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

1. A class D audio amplifier without feedback loop containing a supplyvoltage source (8), an amplifier low-pass filter (14), a power stage (2)controlled by a pulse width modulated signal, a saw-shaped voltagegenerator (4) and a comparator (3), to one of which inputs an audiosignal is sent, while its second input is connected to the adder (6) ofthe compensation circuit of supply voltage influence on the output audiosignal, to which a voltage from a reference voltage source is sent,characterized in that a low-pass filter (9) and a high-pass filter (10)are connected to the supply voltage source (8), and the referencevoltage source (12) is connected to an inverting circuit (11), whoseinput is connected to the low-pass filter (2) output, while thehigh-pass filter (10) output and the output of the inverting circuit(11) are connected to a multiplier (7), whose output is connected to theinput of another multiplier (5), whose second input is connected to thesaw-shaped voltage generator (4), and the multiplier (5) output isconnected to one input of the adder (6), whose second input is connectedto the saw-shaped voltage generator (4).
 2. The class D audio amplifier,according to claim 1, characterized in that the output signal v₀(t) ofthe inverting circuit (11) sent to the multiplier (7) input, which is amodified constant of the supply voltage, is expressed by a formulav₀(t)=k₁×V_(DCref)/[k₂×v_(i)(t)], where V_(DCref) is the voltage of thereference source, v_(i)(t) is a slow-changing signal on the low-passfilter (9) output, and the coefficient k₁ε<0.5; 2.0> and the coefficientk₂ε<0.2; 1.5>.
 3. The class D audio amplifier, according to claim 2,characterized in that the output signal of the multiplier (7), which isthe error signal e(t), sent to the multiplier (5), is expressed by theformula e(t)=k₃×v₀(t)×v_(ii)(t), where v₀(t) is a modified supplyvoltage constant, v_(ii)(t) is a fast-changing signal on the high-passfilter (10) output, and the coefficient k₃ε<0.8; 10.0>.
 4. The class Daudio amplifier, according to claim 3, characterized in that the outputsignal V_(CM)(t) of the adder (6), which is the corrected carrier wavesignal, sent to one input of the comparator (3), is expressed by theformula V_(CM)(t)=k₄×V_(C)(t)×[1/k₅+e(t)], where V_(C)(t) is a highfrequency carrier wave generated by the generator (4), e(t) is the errorsignal, and the coefficient k₄ε<0.2; 1.5> and the coefficient k₅ε<0.2;3.0>.
 5. A method of compensation of supply voltage influence on theoutput audio signal in an audio amplifier, which contains a saw-shapedsignal generator and a comparator making use of pulse width modulation,and which is powered from a power supply, and to whose input an audiosignal is sent, and whose second input is connected to an adder of acompensation circuit of supply voltage influence on the output audiosignal, to which a voltage from a reference voltage source is sent,characterized in that from the power supply source (8) a fast-changingsignal v_(ii)(t) and a slow-changing signal v_(i)(t) are extracted andthen the slow-changing signal v_(i)(t) is inverted and multiplied by thea value of a reference supply voltage V_(DCref), which results in anoutput signal v₀(t), which then is multiplied by a fast-changing signalv_(ii)(t), which results in an error signal e(t), which then ismultiplied by a saw-shaped signal V_(C)(t) from the generator (4), andthe resulting signal is added to a saw-shaped signal V_(C)(t) and as acorrected carrier wave V_(CM)(t) is sent to one of the inputs of thecomparator (3), which makes use of pulse width modulation, and to itssecond input the audio signal is sent.
 6. The method of compensation ofsupply voltage influence, according to claim 5, characterized in thatthe output signal v₀(t) of the inverting circuit (11) sent to themultiplier (7) input, which is a modified constant of the supplyvoltage, is expressed by a formula v₀(t)=k₁×V_(DCref)/[k₂×v_(i)(t)],where V_(DCref) is the voltage of the reference source, v_(i)(t) is aslow-changing signal on the low-pass filter (9) output, and thecoefficient k₁ takes the values from the range <0.5; 2.0> and thecoefficient k₂ takes the values from the range <0.2; 1.5>.
 7. The methodof compensation of supply voltage influence, according to claim 6,characterized in that the output signal of the multiplier (7), which isthe error signal e(t), sent to the multiplier (5), is expressed by theformula e(t)=k₃×v₀(t)×v_(ii)(t), where v₀(t) is a modified supplyvoltage constant, v_(ii)(t) is a fast-changing signal on the high-passfilter (10) output, and the coefficient k₃ takes the values from therange <0.8; 10.0>.
 8. The method of compensation of supply voltageinfluence, according to claim 7, characterized in that the output signalV_(CM)(t) of the adder (6), which is the corrected carrier wave signal,sent to one input of the comparator (3), is expressed by the formulaV_(CM)(t)=k₄×V_(C)(t)×[1/k₅+e(t)], where V_(C)(t) is a high frequencycarrier wave generated by the generator (4), e(t) is the error signal,and the coefficient k₄ takes the values from the range <0.2; 1.5> andthe coefficient k₅ takes the values from the range <0.2; 3.0>.
 9. Aclass D audio amplifier comprising a comparator having a first inputcoupled to an audio signal and a second input; a supply voltage source;a power stage coupled to the supply voltage source and to an amplifierloaded by a loudspeaker, the power stage controlled by a pulse-widthmodulated signal received from the comparator; a saw-shaped voltagegenerator for generating a carrier signal of high frequency; and acompensation circuit for providing a modified carrier signal to thesecond input of the comparator, whereby the modified carrier signal isgenerated by the compensation circuit as a function of the carriersignal and the supply voltage source signal, which are sent to inputs ofthe compensation circuit.
 10. The class D audio amplifier according toclaim 9, characterized in that the compensation circuit comprises alow-pass filter coupled to the supply voltage source and generating aslow-changing signal at a low-pass filter output; a high-pass filtercoupled to the supply voltage source and generating a fast-changingsignal at a high-pass filter output; a reference voltage source; aninverting circuit coupled to the reference voltage source and to thelow-pass filter output and generating an inverting circuit signal at aninverting circuit output; a first multiplier coupled to the high-passfilter output and the inverting circuit output and generating an errorsignal at an output of the first multiplier; a second multiplier coupledto the saw-shaped voltage generator and the output of the firstmultiplier and generating a signal of the second multiplier at an outputof the second multiplier; and an adder connected to the saw-shapedvoltage generator and the output of the second multiplier and generatinga corrected carrier wave sent to the second input of the comparator.